An electrical heating catheter is a tube typically between 1 and 10 millimeters in diameter used for insertion into biological structure and equipped at the distal end with one or more electrodes and at the proximal end with electrical connectors for application of electric power. Electrical heating catheters are useful in many medical applications, e.g., for hyperthermia treatment of cancer or for cardiac ablation of arithmogenic tissue in the endocardium. In such medical applications it is desirable to maintain a fairly uniform generation of heat in a controlled volume of tissue adjoining the catheter.
A radiofrequency (RF) cardiac ablation catheter is presented here as a preferred embodiment. Catheter ablation is a non-surgical method of destroying an arrhythmogenic focus tissue in the endocardium. Typically, an ablation catheter is introduced percutaneously and advanced under fluoroscopic guidance into the left heart ventricle. It is manipulated until the site of the earliest activation is found, indicating the location of problem tissue. RF power is then applied to the distal catheter electrode. The heat in the vicinity of the electrode destroys the cardiac tissue responsible for the arrhythmia.
The temperature boundary between viable and non-viable tissue is approximately 48.degree. Centigrade. Tissue heated to a temperature above 48.degree. C. is non viable and defines the ablation volume. For therapeutic effectiveness the ablation volume must extend a few millimeters into the endocardium and must have a surface cross-section of at least a few millimeters square. The objective is to elevate the basal tissue temperature, generally at 37.degree. C., fairly uniformly to the ablation temperature above 48.degree. C., keeping however the hottest tissue temperature below 100.degree. C. At approximately 100.degree. C. charring and desiccation take place which seriously modifies the electrical conductivity of blood and tissue, and causes an increase in the overall electrical impedance of the electrical heating circuit and a drop in the power delivery to the tissue. Charring is particularly troublesome at the surface of the catheter electrode, since the catheter must be removed and cleaned before the procedure can continue.
In cardiac ablation catheters, the operative electrode is typically metallic and is located on a distal-tip end of the device. This electrode which serves as the heating applicator is referred to as an active electrode. Such an active electrode is the source of electrical or electromagnetic field, which causes heating of neighboring tissue. Even though no significant amount of heat is generated in the electrode itself, adjacently heated endocardial tissue heats the electrode via heat conduction through the tissue.
The field generated by the active electrode also heats the rapid blood flow in the heart chamber, which however very effectively carries away this generated heat so that the flowing blood temperature, except for the boundary layer, stays close to the basal temperature. Some cooling of the catheter tip takes place due to forced convective cooling caused by fast flowing blood in the heart chamber. The active electrode temperature is the result of the balance between such conductive heating and convective cooling.
In one preferred embodiment, the heating and cooling of the active electrode, for the purpose of tip temperature regulation and improved tissue temperature control, is carried out by covering the active electrode with thermally conductive and electrically insulating material. It is therefore appropriate to review the use of dielectric coatings in catheter art and to point out the fundamental difference between the preferred embodiments and the catheter microwave radiator art.
Frequencies for powering heating catheters range from dc to microwaves. It is customary to divide the spectrum of operating frequencies into conductive and radiative regions because of fundamental differences of implementation in these two regions. The dividing frequency between the two regions depends on the characteristic admittance of the tissue surrounding the active electrode: The conductive region is defined by operating frequencies where the conductivity term dominates. Alternatively, in the radiative region the dielectric term dominates. For blood and muscle, the dividing frequency is approximately at 400 MHz. The conductive region corresponds to dc to 400 MHz; the radiative region corresponds to microwave frequencies above 400 MHz.
Implementation of catheter heating applicators for the radiative and the conductive region is quite different. In the radiative region, the heating applicator acts as an antenna causing electromagnetic wave propagation into the tissue. The art of radiative catheter heating applicators, relying on wave propagation, is quite rich, e.g., dipole antennas, helical radiators, and resonators.
An example of a radiative catheter heating applicator, using a resonator, is described in the UK Patent Application GB 2 122 092 A by J. R. James, R. H. Johnson, A. Henderson, and M. H. Ponting. James matches a wave impedance of a resonant radiator, in a mode corresponding to multiples of a quarter wavelength, to a wave impedance of surrounding tissue, by appropriate selection of (1) an electrode coating size, (2) a coating dielectric constant, and (3) a coating magnetic permeability. Neither thermal nor electrical conductivity of the coating is a part of James's design. It should also be noted that the coating in James has both uniform thickness and uniform dielectric properties.
In the conductive region, there is no electromagnetic wave propagation and so techniques relying on wavelength resonance and matching of wave impedance are not applicable. Also in the conductive region, especially in the lower frequencies below 1 MHz, the capacitive impedance of a typical dielectric coating is so high, in comparison with the tissue impedance, that a dielectric coating, in effect, prevents a current flow into the tissue.
Typical state of the art catheter heating applicators for the conductive region, such as the United States Catheter Industries (USCI) catheter shown in FIG. 1, and described in detail later, has an active electrode at the end of the catheter tube and possibly ring electrode or electrodes around the diameter of the tube. Electrodes are connected to the proximal end with a thin, flexible wire.
One undesirable feature associated with such a state-of-the art catheter is a formation of hot spots along the circular junction of the active electrode with the insulating catheter tube due to a sudden transition of electrical properties at the boundary. For example, an article "Catheter ablation without fulguration: Design and performance of a new system", A. J. Ahsan, D. Cunningham, E. Rowland, and A. F. Rickards. PACE. Vol. 12, Part II, January: 001-005, 1989 ("Ahsan") shows such formation of hot spots along the circular junction of the active electrode with the insulating catheter tube. To remedy this problem, Ahsan suggests a cylindrical electrode with hemispherical termination at both ends. The problem with Ahsan's solution is that the electrical connection to such an electrode breaks the smoothness of the surface and so generates a hot spot at the junction of the wire with the hemisphere.
The other undesirable feature of the state-of-the art catheter heating applicators is that there is no provision for cooling of the active electrode and as a result, maximum temperature is reached at the electrode and the resultant charring frequently fouls the electrode during a procedure. The temperature profile taken along the axis of the catheter as it extends into the tissue, similar to that shown by the dashed line in Graph (A) in FIG. 2, has been studied by D. E. Haines and D. D. Watson. (PACE Vol. 12, June: 962-976, 1989) and is described in some detail later. It will suffice here to observe that state-of-the-art catheters exhibit the highest temperature at the active electrode and therefore worst charring occurs at the active electrode-tissue boundary.